1. Field of the Invention
This invention relates generally to strong, porous, thin, tubular improved electrical connectors and supports for solid oxide electrolyte fuel cells in a fuel cell generator.
2. Background Information
Square pitched, series-parallel interconnection of solid oxide fuel cells is well known, and taught in U.S. Pat. Nos. 4,490,444 and 4,833,045(Isenberg and Pollack et al., respectively). The fuel cells used usually contain a self-supported air electrode tube, where the air electrode is covered over about 300 degrees by a solid electrolyte film. Thus, there is a 60 degree wide axial strip down the length of the cell. This remaining 60 degrees of air electrode surface is covered by an interconnection strip, usually a lanthanum-chromite. As a top layer, fuel electrode covers the solid electrolyte over about 280 degrees of the electrolyte surface.
These cylindrical cells are placed in a square pitch, series-parallel connected array, wherein the air electrode of one cell is connected to the fuel electrode of the adjacent series-connected cell by a plated interconnection strip and a strip of 80% to 95% porous sintered nickel felt, which has a continuous core of fiber through its thickness, which is about 0.1 inch (0.25 cm). Other nickel felts provide parallel connections between the fuel electrodes of adjacent cells. The series path is essential for the generation of a practical DC stack voltage. The parallel connections provide paths by which the current can circumnegotiate any defective open circuit cells. Fuel flows axially in the passages forced between the groups of cells. This has been the standard design for over fifteen years.
In this standard design, the primary subassemblies from which a solid oxide fuel cell generator is formed are cell bundles. Presently, bundles contain twenty-four cells on a 8xc3x973 cell matrix. Eight cells are series connected to form one row of a three-row bundle. The three rows are connected in parallel through the connection of each cells in the row with the adjacent cell in the next row. Between the nickel plated lanthanum-chromite interconnection strip of one cell and the nickel fuel electrode of the next cell in a row, any two cells are presently series connected by a nickel felt of a rectangular cross-section (approximately 6.3 mmxc3x976.3 mm). These felts are pressed to a thickness of between about 0.1 inch (0.25 cm) and about 0.25 inch (0.63 cm) and are initially about 80% to 95% porous. Parallel connection is also currently accomplished by similar felt strips. In this case, the felts connect the fuel electrodes of adjacent cells. Along the length of a cell, eight felts of 185-mm length are used to form a series connection, and four felts of 185-mm length are used to accomplish a parallel connection. A total of 280 felt strips are used per bundle. This means of electrical connection is effective; however, it is costly in terms of materials and is very labor intensive. Furthermore, this arrangement is not conducive to automation.
Improvements to this standard design have been suggested. Reichner, in U.S. Pat. No. 4,876,163, disclosed spiral or folded row connections within a cylindrical generator, using U-shaped connections. This design, however, substantially decreases gaseous fuel flow between the outer electrodes of the cells. U.S. Pat. No. 5,273,838 (Draper et al.) eliminated one nickel felt connector from each group of four cells, where alternate cells of a first row had no electrical connection of their interconnections to cells in an adjacent row. This design helped to eliminate the potential for bowing when using newer, longer one meter cells. This design may, however, decrease the overall strength of the twenty-four cell subassemblies.
In an attempt to simplify generator design and reduce assembling costs, DiCroce et al., in U.S. Pat. No. 5,258,240, taught a thick, flat-backed, porous metal fiber felt connector strip, having a crown portion of metallic fiber felt conforming to the surface of its contacting fuel cell. These porous felt connectors could be used as a series of thin strips across a small part of the fuel cell length, or as a porous sheet extending along the entire axial length of the fuel cells. In order to provide structural integrity, since there are no side connections, a plurality of cells would have to be laminated to provide a thickness of 0.125 inch (0.62 cm), thereby reducing porosity to about 5 to 10%. The strips could also be made of a solid nickel foil or a composite of foil and porous felt; they could also have two opposing fuel cell conforming surfaces, as shown in FIG. 3 of that patent. In U.S. Pat. No. 4,874,678 (Reichner) felts were used as connectors between elongated type tubular fuel cells having a plurality of axial interior oxidant feed tubes.
The use of fibrous felts still allowed potential densification during prolonged use and were difficult to fashion to exact dimensions, while the use of foils did not provide adequate strength.
In a rather radical fuel cell design, Isenberg, in U.S. Pat. No. 4,728,584, taught spaced apart fuel cells interconnected by solid wire connectors. In fuel cell bundles where close spacing is required, the solid wire would cause significant thermal expansion and pressure problems, leading to fuel cell cracking. Draper et al., in U.S. patent application Ser. No. 09/631,096 (filed on Aug. 3, 2000, Attorney Docket No. 00E7705US, 283139-01096), now U.S. Pat. No. 6,379,831, taught expanded nickel mesh corrugated into crown portions and shoulder portions and disposed across a complete layer of fuel cells. While providing excellent support and resilience, this design required substantial amounts of forming and molding, adding significantly to the costs of the fuel cell generator.
What is needed is a highly porous nickel support made of a single piece, disposed between individual fuel cells, to connect all contacting fuel cells electrically.
Therefore, it is a main object of this invention to provide a thin, strong, porous electrical connector for solid oxide electrolyte fuel cells in a fuel cell generator.
It is also a main object of this invention to provide an improved, inexpensive connector for fuel cells in a fuel cell generator that will retain its resilience throughout its life.
These and other objects of the invention are accomplished by providing a solid oxide fuel cell assembly comprising at least columns of fuel cells, each fuel cell having an outer interconnection and an outer electrode, where the fuel cells are disposed next to each other with rolled, porous, hollow, electrically conducting metal mesh conductors between fuel cells, connecting the fuel cells at least in a series along columns, and where there are no metal felt connections between any fuel cells. The mesh is preferably plural rolled and can be expanded metal mesh or woven metal mesh, preferably nickel or copper metal mesh having from two to six layers, with the rolled structure mentioned previously. This invention is meant to cover the tubular type fuel cells as well as the elongated type fuel cells shown in Reichner U.S. Pat. No. 4,874,678, where the term tubular fuel cell is meant to also include this latter type design.
This provides a conductor that accommodates differential thermal expansion shear stress and provides shock resistance. It also provides a very strong, springy, thin, and stiff electrical connector support for tubular solid oxide fuel cells, having high mechanical durability and resilience, and which will last substantially longer than other types of connectors and is inexpensive to manufacture. It also has a low resistance loss through its thickness.